The present invention relates to a developing device for use in an image forming apparatus and having a developing roller, developing sleeve or similar developer carrier for supplying a one-component developer, or toner, to a latent image electrostatically formed on a photoconductive element or similar image carrier.
In an image forming apparatus implemented by an electrophotographic procedure such as a copier, laser beam printer or facsimile machine, a latent image representative of a document is electrostatically formed on a photoconductive element or similar image carrier. The apparatus has a developing device in which a developing roller, developing sleeve or similar developer carrier is accommodated. The developer carrier supplies one-component developer, i.e., toner to the latent image to develop the latent image on the photoconductive element. A developer supplying member in the form of a roller, for example, is held in contact or located in close proximity to a developer supplying region of the developer carrier. A regulating member implemented as a blade or a roller regulates the thickness of a developer layer formed on the developer carrier and is held in contact with the developer carrier in a regulating region which is defined downstream of the developer supplying region. In the developer supplying region, the developing supplying member supplies a fresh toner from a toner storing section to the developer carrier. The toner deposited on the image carrier is leveled by the regulating member to form a layer having a predetermined small thickness. The regulated toner layer is fed into a developing region located downstream of the regulating region to be selectively supplied to the latent image.
The developer carrier in the form of a roller or a sleeve is made up of a conductive base and a dielectric layer formed on the base for adjusting the electric field for development. The developer carrier is pressed against the photoconductive element or similar image carrier in order to free the developing device from the need for accurate positioning control. The dielectric layer is generally implemented by a material having elasticity such as polyester urethane or similar urethane-based composition. A problem with a prior art developing device having such a developer carrier is that the developing characteristic is susceptible to developing linear velocity which is determined by the relationship between the moving speed of the image carrier and that of the developer carrier, degrading the developing quality. Specifically, in an ordinary developing device, a predetermined electric field for development is set up in the developing region defined between the image carrier and the developer carrier, whereby a force acts on the charge deposited on the toner particles existing in the developing region. Whether or not the toner particles are allowed for development is dependent on the intensity of the force acting on the charge as mentioned above. The materials constituting the developer carrier, especially the material of the dielectric layer, have great influence on the electric field in the developing region. Specifically, the electric field noticeably fluctuates when the dielectric layer of the developer carrier is implemented by polyurethane or similar urethane-based composition or any other material whose dielectric relaxation characteristic is poor. In such a condition, the developing characteristic is critically effected to prevent the developing device from performing desirable development constantly.